Data communication systems typically involve a transmitter, a receiver, and a transmission path between the transmitter and receiver. The transmission path may be air or cables (wire or optical fiber). Data is frequently transmitted in a data communication system in a form which requires the receiver to be synchronized with the transmitter. For example, when data is spread in the frequency and/or time domains during transmission, the receiver must be synchronized to the transmitter in order to accurately recover the transmitted data.
A synchronizer typically uses a synchronization signal which is transmitted by the transmitter along with data. The synchronizer synchronizes the receiver to the synchronization signal and, when synchronization is acquired, the receiver is able to recover the data.
U.S. Pat. No. 6,304,619 discloses an up chirp and a down chirp which may be transmitted as a synchronization signal. A reference up chirp and a reference down chirp are correlated to the received signal in order to generate an up correlation peak index between the transmitted up chirp and the reference up chirp and a down correlation peak index between the transmitted down chirp and the reference down chirp. A frequency error is calculated based upon the difference between the up correlation peak index and the down correlation peak index, and a timing error is determined as the average of the up correlation peak index and the down correlation peak index. The frequency and timing errors are then used to acquire synchronization.
According to the arrangement disclosed in the aforementioned patent, the up chirp and the down chirp are transmitted sequentially in time, as shown in FIG. 1. Thus, the frequency of the up chirp increases from a frequency f0 at a time t0 to a frequency fN at a time tN, and the frequency of the down chirp decreases from the frequency fN at the time tN to the frequency f0 at a time t2N.
The present invention, on the other hand, is directed to a compound chirp which combines the attributes of both an up chirp component and a down chirp component but which, as shown in FIG. 2, occupies a shorter time interval than if the up chirp component and the down chirp component were transmitted sequentially. That is, as shown in FIG. 2, the frequency of the compound chirp according to one embodiment of the present invention (i) increases from a frequency f0 at a time t0 to a frequency f1 at a time tM, (ii) decreases from a frequency f2 at the time t0 to a frequency f1″ at the time tM, (iii) increases from a frequency f2″ at the time t0 to a frequency f3 at the time tM, (iv) decreases from a frequency f4 at the time t0 to a frequency f3″ at the time tM, (v) increases from a frequency f4″ at the time t0 to a frequency f5 at the time tM, (vi) decreases from a frequency f6 at the time t0 to a frequency f5″ at the time tM, (vii) increases from a frequency f6″ at the time t0 to a frequency f7 at the time tM, and (viii) decreases from a frequency f8 at the time t0 to a frequency f7″ at the time tM. The time tM may be shorter than, equal to, or longer than the time tN. As shown in FIG. 2, this compound chirp appears to be folded or pleated. The compound chirp of the present invention improves synchronization response times because it has a shorter duration than a non-folded chirp spanning the same bandwidth.